Pump systems are used in the industries and in public utility services, among other things. In industrial applications, pump systems are in most cases used in connection with production processes, while they relate to transfer of pure water, rain water and waste water in municipal engineering. In conjunction with starting up of a pump system, there can be a situation that pipes into which a pump is intended to feed liquid are not filled with liquid at the beginning of the starting phase. This kind of situation is repeatedly present e.g. with a movable irrigation pump system. When an irrigation pump system is moved from one place to a new place there is usually a situation that in the new place the pipes are empty or incompletely filled. Another application having frequent start ups with empty or incompletely filled pipes is a snow-machine in which there is a need to empty the pipes after use in order to avoid freezing in the pipes.
Pump systems used for liquid transfer usually consist of an electrically driven pump. The electric drive consists of a suitable power supply circuit, an electric motor and a control unit suitable for controlling and/or adjusting this. The pump operates as a mechanical load on the electric drive. A frequently used electric motor in pump systems is an alternating-current motor, especially an induction motor. The control unit used in an alternating current motor often consists of a frequency converter because of the benefits gained by this. Rotation speeds of the electric motor and the pump are adjusted by the frequency converter, which converts the frequency of the voltage supplied to the motor. The frequency converter, again, is adjusted by appropriate electric control signals.
Controlling the speed of a pump during a start up when pipes connected to a flow output of the pump are empty or incompletely filled is a challenging task from the viewpoint of avoiding pressure peaks in the pipes at the moment when the pipes get full of liquid. This is due to a fact that a counter-pressure versus flow rate characteristics that is prevailing at the flow output of the pump is rapidly changed when the pipes get full of liquid.